The invention relates to metal-protecting, electrically insulating compositions, films and more particularly to sheet or film material applied to substrates at elevated temperature to provide primary insulation, such as bus bar insulation, on electrical conductors.
Bus bars are well known as interconnection structures for use with electricity distribution networks and related switchgear. Portions of a bus bar, along its length, provide sites for electrical connection to bare metal. Other portions of the bar have an overlying protective insulating coating. There is a variety of insulating coatings that may be applied to metal substrates, including e.g. bus bars and electrical current carrying wires, using several alternate methods. Conventional coatings for metal may be curable compositions such as electrically insulating resin coatings including filled polysiloxane compositions, poly(vinylidene fluoride) compositions and epoxy resin compositions. Japanese Patent No. JP 11203944 provides a general description of insulating resin coatings for bus bars. Publications describing curable polysiloxane insulating compositions include published application WO 9843251, U.S. Pat. No. 5,641,827 and French Patent No. FR 2644619 that describes a filled silicone elastomer, wire insulation, cured at 80xc2x0 C. for 20 min. Poly(vinylidene fluoride) arc resistant compositions may be used to insulate electrical switches as described by Japanese Patent No. JP 57023377. Epoxy resin coatings for metal may be applied from solution as described in Annual Tech. Conf., Soc. Plast. Eng., Tech. Pap., 27th (1969), 15 34-8. Publ. Soc. Plast. Eng., Inc., Stamford, Conn. They may also be electrophoretically deposited on to copper conductors according to JP 59134508, or an epoxy resin coating may be applied to a metal surface by fusion bonding using fluidized bed techniques. Evidence exists for epoxy coating formation using fusion bonding by reference to U.S. Pat. No. 4,885,187 and Proc. Electr./Electron. Insul. Conf., 12th (1975) 165-70 Publ. IEEE, New York, N.Y., which describes the benefits of powdered epoxy, fusion bonded coatings compared to polyethylene terephthalate tape for protection and insulation of bus bars.
Several other information sources describe the use of tape or film or wrap, for bus bar protection and insulation. For example, German Patent No. DE 19710881, U.S. Pat. No. 3,504,103 and published application WO 96/25292 use adhesive coated wrapping materials for bus bar covers. Tubular covers may also be used for this purpose as described in German Patent No. DE 1640778. In some cases the tubular covers may be heated to a selected temperature at which shrinkage occurs so that the heat-shrink tube establishes more intimate contact with the metal surface. An alternative process for wrapping a metal surface involves the application of a band of felt or cloth to a curable layer of epoxy resin coated on a metal surface. Curing of the epoxy layer then bonds the felt or cloth to the metal.
The use of adhesive coated wrap materials or special preparation of metal surfaces to promote bonding of insulation to the metal represent time consuming and costly processes for protecting and electrically insulating metal substrates. A need exists for an insulating cover composition, sheet or tape that is relatively convenient to manufacture and bonds to metal using a simple method of application that may be readily automated.
The present invention provides an extrudable, electrically insulating, optionally flame retarding, composition and film possessing latent crosslinking and bonding capability. Extrudable, insulating compositions according to the present invention comprise a crosslinkable polyolefin matrix material containing a co-bonding agent for metal. Prevention of premature crosslinking of either the polyolefin or the co-bonding agent requires care during both the material compounding and film forming stages of manufacture. Compounding and film extrusion require process temperatures below the decomposition temperature of curative materials and below the temperature used to bond an electrically insulating film to a metal substrate. A co-bonding agent typically cures by a thermoset mechanism. Polyolefin crosslinking also proceeds at elevated temperature. Once curing or crosslinking of either compound or film is initiated the process cannot be reversed.
Films of the insulating composition according to the present invention bond to pre-heated metal at a temperature sufficiently elevated to activate the co-bonding agent and promote the crosslinking reaction of the polyolefin polymer. Optionally, the insulating composition may include flame retardants, arc suppressants, extrusion aids and other additives to meet the requirements of specific applications. Preferably, extruded material may be applied as film or tape to bus bars using an automated process.
More particularly, the present invention provides an electrically insulating composition for bonding to metal. The insulating composition comprises a polyolefin polymer adapted for elevated temperature crosslinking and a co-bonding agent. The co-bonding agent may be mixed with the polyolefin polymer in an amount up to about 60 parts of the co-bonding agent to 100 parts of the polyolefin polymer. Preferably, the insulating composition bonds to a metal pre-heated to elevated temperatures from about 140xc2x0 C. to about 190xc2x0 C., during crosslinking of the polyolefin and curing of the co-bonding agent. The resulting structure has a lap-shear bond strength between the electrically insulating composition and a metal from about 3.3 MPa to about 12 MPa.
An insulating composition according to the present invention may be applied for bonding to a metal in a variety of material forms. After curing, the insulating composition provides a barrier layer over the metal. The layer is formed preferably by applying the insulating composition in the form of a film or sheet material.
The present invention also includes a method for insulating a metal substrate wherein the metal substrate is first heated to an elevated temperature in a range from about 140xc2x0 C. to about 190xc2x0 C. At least one sheet of an electrically insulating composition is applied to the heated metal substrate. Optionally the sheet extends over the edges of the metal substrate for formation of an overlapped seam. The electrically insulating composition comprises a polyolefin polymer suitable for elevated temperature crosslinking, and a co-bonding agent mixed with the polyolefin polymer in an amount up to about 60 parts of the co-bonding agent to 100 parts of the polyolefin polymer. The electrically insulating composition cures in contact with the metal substrate at the elevated temperature to provide a lap-shear bond strength between the electrically insulating composition and the metal substrate of from about 3.3 MPa to about 12 MPa. Application of pressure to the sheet causes it to conform to the contours of at least a portion of the metal substrate. This provides an insulation-protected metal substrate which undergoes thermal conditioning to further bond the sheet to the metal substrate during crosslinking of the polyolefin and curing of the co-bonding agent.
Overlapping for seam formation with a single sheet or between multiple electrically insulating sheets represents one method for insulating electrically conducting substrates. Use of this method ensures full coverage of the conductor by the insulating composition. An alternative method produces conducting substrates covered by sheets of insulating material having seams in an abutting relationship. In this case, the process requires precise application of sheet material to produce insulated substrates without gaps in the vicinity of the abutted seams.